As one skilled in this technology appreciates, the efficiency of the gas turbine engine such as a microturbine engine increases as the temperature of the engine working medium at the turbine increase. However, because of the limitations of the structural integrity of the material of the turbine, it is necessary to cool the turbine in order to take advantage of the high temperature of the engine working medium. In a microturbine engine the turbine is cooled by allowing a portion of the air in the compressor to leak and fill the cavity between the compressor and turbine to define a buffer zone to prevent back flow of high temperature engine working medium from the turbine to migrate into this cavity and to utilize this air to cool the turbine. This has been the manner in which the turbine has been cooled in those types of microturbine engines where the compressor rotor and turbine rotor are mounted back to back. Since the cooling flow is predicated on the amount of leakage flow allowed to migrate from the compressor, it is quite apparent that this flow is not a controlled amount and can vary from engine to engine. The impact of this variance is that the efficiency of each of the engines coming off of the assembly line varies. Inasmuch as the efficiency of the engine is one of the parameters that is relied on in the engine's specification, it is abundantly important to maintain a predictable efficiency for each of these engines. In accordance with this invention, the leakage flow from the compressor is controlled by sealing off the path between the turbine and compressor and locating discrete holes to meter the leakage flow in order to regulate the amount of total flow of cooling air being taken from the compressor and hence, eliminates or at least reduces efficiency scatter for all of the engines being manufactured. Because of the varying load conditions the microturbine encounters, the shaft interconnecting the compressor and turbine has the tendency of deviating from its given path and these excursions interfere with maintaining proper clearances for obtaining good or even adequate sealing. This invention has found that brush seals have the advantage over other types of seals because they handle excursions without adversely affecting the sealing capability of the seal.
The prior art is replete with disclosures of brush seals for sealing rotary shafts to prevent the high pressure fluid on one side of the seal to leak into the low pressure fluid on the other side of the seal. As disclosed in U.S. Pat. No. 5,639,211 granted to Bintz on Jun. 17, 1997, a brush seal is mounted on the foot of the stator vane of a gas turbine engine and bears against the rotating shaft so as to seal the upstream side of the stator from the downstream side of the stator vanes. That is to say, the brush seal serves to prevent the higher pressure engine working medium upstream of the stator vanes to migrate to the lower pressure side on the downstream side of the stator vanes. In this configuration the backing plate for the bristles of the brush seal is a split ring that fits onto the foot of the stator vane and the front plate or segmented retainer is mounted back to back with the backing plate and the bristles are sandwiched therebetween. As is the case of most of the brush seals, the backing plate is radially longer than the fore plate and is mounted so that the high pressure of the fluid forces the most rearward bristles against the longer extending portion of the retaining mechanism. The bristles may be oriented at an angle relative to the rotational direction of the shaft or may be chamfered to assure maximum wear characteristics. Typically, in a jet engine where the seals are subjected to a hostile environment, the brush seals are made from a relative stiff material that is capable of withstanding substantially high temperatures, say in the range of 1400 degrees Fahrenheit to 2000° F. or higher. An example of suitable material is cobalt alloy wire although any other suitable high temperature resistance material could be used. In many of the brush assemblies the tufts or highly packed bristles are typically welded or brazed on one end to the backing plate while the other end extends radially to engage the shaft in a cantilever fashion.
U.S. Pat. No. 6,250,879 granted to Lampes on Jun. 26, 2001 and incorporated herein by reference discloses an example of a brush seal utilized to seal the high temperature fluid adjacent the turbine of a gas turbine engine and the turbine static support structure supporting the turbine rotor.
U.S. Pat. No. 5,480,162 granted to Beeman, Jr. on Jan. 2, 1996 and incorporated herein by reference is another example of a brush seal mounted between static parts of a gas turbine engine and is subjected to extremely hot temperatures. This seal serves to prevent combustion gases in the combustor from escaping from the vanes of the stator vanes feeding engine working medium to the turbine while avoiding loses of engine working medium so as to maximize TSFC, thrust specific fuel consumption, and consequently, avoid a deficit in fuel consumption so as to maintain a high engine efficiency.
U.S. Pat. No. 6,457,719 granted to Fellenstein, et al and incorporated herein by reference, exemplifies a brush seal that includes a plurality of circumferentially spaced holes in the backing plate to provide a leakage path from the high pressure side to the low pressure side through the bristles. This disclosure is particularly suited for sealing between stationary and rotary components and serves to provide an alternative leakage path so that the bristle loadings in the region adjacent the edge of the backplate is less than what is understood to be the situation in prior known brush seal applications. In this disclosure, since the leakage flow through the bristle pack region between the back plate and the sealing surface is less than those designs prior to the subject disclosure, the probability of deformation of the bristles due to high loadings is reduced.
This invention contemplates judiciously mounting the brush seal/seal plate between the cantilevered mounted compressor/turbine so that the bristles of the brush seal bear against the shaft that is rotary supporting the compressor and turbine in a microturbine engine and define a barrier therebetween. This arrangement serves to control the leakage of the higher pressure compressor air to the back face of the turbine and assures that the lateral excursions of the shaft will not produce spikes of flow variations. This arrangement serves to assure that the brush seals at this location of the engine of all the same model engines being manufactured during a given interval will have the same amount of leakage throughout their operating range and the turbine will operate at relatively the same temperature throughout its operating envelope and hence, the efficiency from engine to engine coming off the assembly line remains constant.